The present invention relates to a water quality management system and more particularly to measurement, monitoring and management of water quality at the end of pipe lines.
Management is made of water by filtering and disinfecting the water from a drainage basin in a purification plant and then distributing only the water that has passed water quality criteria to consumers. Among criteria for evaluating the quality of tap water are residual chlorine (for example, in Japan, 0.1 mg/L or higher for water at taps; 0.4 mg/L or less for tasty water), turbidity (2 degrees or less) and color (5 degrees or less). A conventional practice for maintaining the quality of water involves sampling water from parts of the water distribution pipe line network and checking residual chlorine concentration, turbidity and color manually or by a water monitoring apparatus to determine the amount of chlorine to be added to the water. As described in JP-A-6-320166, another conventional method of controlling the amount of chlorine to be added at the purification plant involves performing fuzzy reasoning based on signals from water quality monitors installed in the water distribution pipe line network and on measured amounts of residual chlorine in a clean water reservoir and a water distribution reservoir at the purification plant to minimize the amount of trihalomethanes. Further, as described in JP-A-7-290040, still another method performs analysis on the water distribution pipe line network to determine the flow and head of the water flowing past nodes (intersections) in the pipe line network and the amount of water taken out at respective nodes and thereby estimate the amount of residual chlorine at each node to adjust the amount of chlorine to be added into the distribution reservoirs provided at the purification plant and in the pipe line network.
The water is supplied from the purification plant to consumers through a distribution main 400 mm or more in diameter which branches into smaller distribution pipes 100-350 mm in diameter. The water in the distribution pipes is required to have residual chlorine concentrations higher than a certain level to prevent multiplication of bacteria. The amount of chlorine in the water decreases exponentially over time due to reaction with organic matters in the water and with pipe walls, and there are variations in the chlorine concentration depending on the route of water supply and the time taken by the water to reach faucets. The rate of reduction in the residual chlorine is affected by the pipe diameter, the presence or absence of lining inside the pipe, and the temperature of water. The chlorine reduction rate is higher in a smaller-pipe distribution network because the reduction rate increases as the pipe diameter decreases and the water temperature becomes high. The time it takes for the water to reach the faucets varies daily and by seasons depending not only on the amount of water used at the faucets but also on the amount of water used downstream. Further, turbidity and color variations are caused by rust due to deteriorated piping and soil particles entering into water through damaged pipes.
Since it is practically impossible to grasp by analysis the state of the small-pipe water distribution network near the consumers beyond the main pipe line branches as well as the water flow in a whole water supply network from the purification plant to the consumers, the control of water quality at the consumers' end of the water supply network is almost impossible.